1. Field of the Invention
The present invention relates to a method of producing a printed circuit board, and in particular to a method of producing a printed circuit board incorporating a resistance element using carbon paste.
2. Related Art
Reduction in size and thickness of electronic components has been demanded in late years, at the same time, electronic components and printed circuit boards on which the electronic components are mounted are also demanded to reduce size thereof by miniaturizing a pattern and increasing its density and to reduce thickness thereof by using a thin material.
A printed circuit board is being developed which incorporates a resistor element formed by printing carbon paste instead of a conventional surface-mounted chip resistor. This printed circuit board reduces an area where chip resister components are mounted. Furthermore, the resistor element using carbon paste can be thinned as compared with a chip resister component, so that the printed circuit board can be reduced in size and thickness.
In a printed circuit board disclosed in Japanese Patent Laid-Open No. 11-340633 (refer to “paragraph number 0014”) and Japanese Patent No. 2547650 (page 2, column 3, line 29 to column 4, line 3) in which a resistance element is formed using carbon paste, problems arise such as printing blur due to a step on a printing surface, printing sag and dispersion in film thickness when the carbon paste is formed using a screen printing method, so that a percent dispersion of resistance of some resistance elements is about 15.
In addition to the above, the resistance is varied by heat and pressure generated in a laminating process when the above printed circuit board is multi-layered and incorporates a resistance element, so that it is difficult to keep the accuracy of the resistance after the resistance element has been incorporated.
In Japanese Patent Laid-Open No. 2006-222110 (refer to “paragraph number 0022”), the surface treatment of an electrode is transferred from paste printing to plating to reduce a step on printing surface, however, measures are not taken for change in resistance in a laminating process.
In a printed circuit board disclosed in Japanese Patent Laid-Open No. 2002-185099 (refer to “paragraph number 0016”) in which a resistance element is formed using carbon paste, a through hole or bottomed hole is filled with the carbon paste to form the resistance element. Filling the through hole or via hole with the carbon paste enables reduction in the dispersion of resistance related to the printing problems posed in Japanese Patent Laid-Open No. 11-340633 and Japanese Patent No. 2547650.
Since neither carbon paste nor copper is stable in resistance because a mutual contact resistance is high, the accuracy of the resistance cannot be assured. The resistance element in which a copper electrode is brought into contact with carbon paste has such a problem that the resistance element is influenced by moisture and oxygen in the air after the resistance element has been formed to increase contact resistance at the interface between copper being an electrode material and the carbon paste, leading to variation in resistance.
Furthermore, an accuracy required for a resistor used as a termination resistor and in a filter in a transmission line is 1% or less, so that an accurate trimming technique using laser processing is needed to improve the accuracy of the resistor. However, a resistance element in which a through hole or via hole is filled with carbon paste has a problem in that the resistance cannot be adjusted using trimming technique such as laser processing after formed.
FIG. 4 is a cross section of a process chart illustrating a method of producing a printed circuit board incorporating a resistance element using carbon paste described in Japanese Patent Laid-Open No. 2002-185099. As illustrated in FIG. 4(1), a so-called double-sided copper clad laminate 24 is prepared in which both sides of an insulating base material 21 of polyimide are provided with a first and a second conductive layer 22 and 23 of copper foil. Through holes 25 and 26 are formed in required positions by drilling or laser processing.
As illustrated in FIG. 4(2), a carbon paste 27 is applied to the through hole 25 by screen printing and thermoset. Then, as illustrated in FIG. 4(3), the first and the second conductive layer 22 and 23, through hole 26 and carbon paste 27 are subjected to a conducting treatment to form a plating film 28.
As illustrated in FIG. 4(4), the first and the second conductive layer 22 and 23 and the plating film 28 are etched by a photo fabrication method to form circuit patterns 29 and 30, thereby obtaining a printed circuit board 31 incorporating a resistance element using the carbon paste.
As described above, the resistance of a resistance element in which a through hole or bottomed hole is filled with carbon paste cannot be adjusted using trimming technique such as laser processing after the resistance element has been formed. For this reason, it is difficult to produce a resistor with an accurate resistance by the carbon-paste filling method.
The present invention has been made in view of the above problems and has for its purpose to provide a method of producing a printed circuit board incorporating a resistance element capable of adjusting resistance after the resistance element has been formed and assuring a high accurate resistance.